Low frequency induction furnace



June 17, 1958 I L. DREYFUS 2,839,652

Low FREQUENCY INDUCTION'FURNACE' v Filed NOV. 19, 1956 IN V EN TOR.

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United States Patent LOW FREQUENCY INDUCTION FURNACE Ludwig Dreyfus, Vasteras, Sweden, assignor to Allmiinna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Application November 19, 1956, Serial No. 622,911

Claims priority, application Sweden November 22, 1955 2 Claims. (Cl. 21910.75)

Induction heating furnaces with cylindric coils are often used for partial heating of the ends of bars or for heating cuttings or billets along their whole length. The heating coil thereat has always an axial length somewhat larger than the length of the work-piece to be heated to full temperature. In high frequency furnaces having equal winding density it is obtained thereby that the workpiece is practically heated uniformly over the whole length (heating of billets), or over the desired partial length (bar heating).

As in the last years the low frequency furnace with 50 or 60 cycles for thorough heating of large, magnetic billets to the Curie point has gained ground, the same structural feature of the furnace coil has been adopted with projecting ends and equal current density along the coil. But it was found that in low frequency furnaces of this type the free ends of the charge always will be colder than the middle portion thereof. It is the purpose of the present invention to over-come this inconvenience.

The invention will be described with reference to the accompanying drawing, in which Fig. 1 shows a coil having a main portion and two projecting portions, and Figs. 2a and 2b show wiring diagrams.

For the understanding of the invention it is necessary first to show why the low frequency furnace and the high frequency furnace are different in the above mentioned respect, for example in heating billets. Considering Fig. 1, in which the main portion of the coil is designated by a and the two projecting portions by b and b, respectively, the iron core by c, and the billet by d, the following will be evident.

If the ends of the billet shall be just as warm as its middle portion, the flux 4: of the coil at the end of the billet has to be the same as in the middle of the billet (rp but only under the assumption that a flux enters axially already at the end faces of the billet, and just the projecting winding portions outside the length of the billet have to generate that flux. The filux of the coil, however, consists of two components, viz. the flux 5 on the surface of the billet including the depth of penetration and the eddy flux da outside, between the billet and the coil. 5 is always in phase agreement with the current 1 of the coil. 4: however, is about 54 later than 5 and I The phase angle of the resulting flux depends on whether the flux or the flux predominates.

In this respect, however, the high frequency furnace and the low frequency furnace work differently. In the high frequency furnace leaving a small depth of penetration r s much smaller than and therefore 2,839,652 Patented June 17, 1958 if the turns N of the projecting winding portions are made sufliciently large, also may have the same value as p with the result that the billet is heated uniformly along its entire length.

The low frequency furnace works quite differently. The fiux of the billet rp is much larger than the eddy flux due to the much greater depth of penetration of the field. It is therefore which generally determines the resulting phase angle of the flux, and it is delayed beyond 45 with respect to I,,,, while the flux m, of the projecting winding portions still is in phase with l If now, as always, the current l is made equal to I by series connection of all the winding portions, p will be about 45 later than p, and the condition for uniform heating, i. e. equal to and in phase with cannot at all be fulfilled.

According to the present invention, this difliculty is overcome in a simple manner. The coil is divided by additional taps into different winding sections, i. e. the main section having N, series connected turns, which cover the length of the billet or generally speaking the length of the charge to be heated, and into one or two projecting sections at the ends of the coil outside the heating length of the charge, this section having N, or (and) N turns.

Phase agreement of the associated flux rp and 4b,, and respectively, is obtained in a simple way by applying to these winding sections voltages E E, and E' respectively, which are in phase agreement to each other. If further, the winding factors of the individual sections are defined by a b 's afa bfb 'bf'b For the reduction in practice it is sufficient to evaluate At billet heating, N is made N' and both sections are suitably series-connected and fed with a voltage The voltages required for the execution of the invention may be obtained in different manners. E, and E -j-E may be taken from a transformer with a transformation ratio E /E +E' These voltages are applied in parallel to the corresponding winding sections. Such a transformer, a so called voltage divider is shown in Fig. 2a. Also a three windings transformer, having two sec ondary windings may be used from which the winding sections N, and N;,+N' are fed, as shown in Fig. 2b.

What I claim is:

1. Low frequency induction furnace comprising a cylindrical coil energized by a single-phase alternating source, at least one of the ends of said coil projecting beyond the length of the charge to be heated characterised in that the coil is divided by additional taps into a main section comprising N turns and covering the length of the charge to be heated and one projecting section comprising N turns for bar heating, and two projecting sections N and N for billet heating and in that the main section and the projecting sections are fed separately by voltages E and E and 'E' respectively in phase agreement of such values that fl i l I bfb afa ihf b wherein f is the winding factor of the sections. 2. Low frequency induction furnace according to claim 1 having two projecting sections N and N char- 4; acterised in that these sections are series-connected and fed with one voltage only (Fig. 2a and 2b).

References Cited in the file of this patent UNITED STATES PATENTS 2,720,576 Lackner Oct. 11, 1955 2,720,577 Lackner Oct. 11, 1955 2,748,240 McArthur May 29, 1956 

